Add comment explaining CSS rule to hide menuitem icons on macOS

chrome/content/zotero-platform/mac/overlay.css

@@ -366,6 +366,8 @@ treechildren::-moz-tree-image {
 	list-style-image: url('chrome://zotero/skin/mac/toolbar-note-add.png');
 }
 
+/* Hide icons on macOS. We use :is() to work around weird behavior in Fx101 where a regular child
+   selector doesn't match the first time the menu is opened. */
 :is(#zotero-collectionmenu, #zotero-itemmenu) > :is(.menuitem-iconic, .menu-iconic) {
 	list-style-image: none !important;
 }